Field of the Invention
The invention concerns a method for correcting image data acquired by operation of an image data acquisition scanner of a magnetic resonance system. The invention also concerns a method for magnetic resonance imaging of a region of an examination subject. The invention further concerns an image correction device for a magnetic resonance measurement. The invention also concerns a magnetic resonance system.
Description of the Prior Art
In a magnetic resonance system, the body that is to be examined is typically exposed to a relatively high basic magnetic field, for example of 1.5 tesla, 3 tesla or 7 tesla, with the use of a basic field magnet. After the basic field is applied, nuclei in the examination subject align themselves along the field because they have a non-zero nuclear magnetic dipole moment, called spin. This collective behavior of the spin system is described by the macroscopic “magnetization”. The macroscopic magnetization is the vector sum of all of the microscopic magnetic moments in the object at a specific location. In addition to the basic field, a magnetic field gradient, by which the magnetic resonance frequency (Larmor frequency) at the respective location is determined, is applied by a gradient coil system. Radio-frequency excitation signals (RF pulses) are then emitted via a radio-frequency transmission system by one or more suitable antennas, with the intended goal of tipping the macroscopic magnetization through a defined flip angle in relation to the magnetic field lines of the basic magnetic field. When such an RF pulse acts on spins that have already been excited, these can be flipped (deflected) into a different angular position or even flipped back into an initial state parallel to the basic magnetic field. During the relaxation of the excited nuclear spins, radio-frequency signals, called magnetic resonance signals, are resonantly emitted, received (detected) by suitable reception antennas (also called magnetic resonance coils or reception coils). The received signals are subsequently demodulated and digitized, and then processed further as data referred to as “raw data”. The acquisition of the magnetic resonance signals takes place in the spatial frequency domain, the so-called “k-space”, the data entry points in k-space being traversed as a function of time along a “gradient trajectory” (also called “k-space trajectory”) defined by the switching of the gradient pulses during a measurement e.g. of a slice. In addition, the RF pulses must be emitted in a coordinated manner as appropriate with respect to time. Following further processing steps, which usually are also dependent on the acquisition method, the desired image data is finally reconstructed from the thus acquired raw data by a two-dimensional Fourier transform. Alternatively, three-dimensional volumes can also be excited and read out in a defined manner in the interim, the raw data in turn being classified after further processing steps into a three-dimensional k-space. A three-dimensional image data volume can then be reconstructed accordingly by a three-dimensional Fourier transform.
In order to control a magnetic resonance tomography system during the measurement, it is common practice to use specific predefined pulse sequences, i.e. strings of defined RF pulses as well as gradient pulses in different directions and readout windows, during which time the reception antennas are switched to reception mode and the magnetic resonance signals are received and processed. Such sequences are parameterized in advance for a desired examination, for example a specific contrast of the calculated images, with the use of a scan specification known as a measurement protocol. The measurement protocol may also contain further control data for the measurement. In this regard there are a multiplicity of magnetic resonance sequence techniques in accordance with which pulse sequences may be constructed.
Particularly when high field strengths of 3 tesla or more are used in MR measurements, local variations in the radio-frequency field, also called the B1 field, can occur that are caused by electrical or dielectric effects. These variations can lead to inhomogeneities in the measured signal distribution, which has an influence on the quality of the reconstructed images acquired, and consequently have an impact on the diagnostic quality. Such variations are generally dependent on the shape and nature of the object being examined, for which reason different variations in the B1 field may be observed in different patients and in different regions of the body.
In this connection, the relevant effects can be subdivided into two categories: Firstly, a variation in the B1 field present in the transmit case causes an inhomogeneous distribution of the achieved flip angles. Phenomena associated therewith are referred to in the following as TX effects. Secondly, local variations in the reception sensitivity of reception coils are also observed, even in the case of such reception coils which are configured as volume coils. The phenomena associated therewith are referred to in the following as RX effects and owing to the reciprocity principle known in MR imaging are also linked with variations in the B1 field.
The symmetry of parts of the body is taken into account in many clinical diagnostic questions. This is the case, for example, in many examinations in the head region. In this regard an asymmetric signal distribution, and consequently an asymmetry in the reconstructed MR image (e.g. in relation to front/back or left/right), may point to a medical disorder. If asymmetric signal distributions are brought about by the above-described TX or RX effects, this can lead to a false indication of a medical disorder, or at least have a negative effect on the image quality.
In transverse head examinations, for example, a disruptive signal overshoot may occur in the right-hand half of the image even though the head is relatively symmetrical (referred to the central sagittal plane) and for that reason the clinical observer would also expect symmetrical images.
Local coils (or surface coils) can exhibit significantly inhomogeneous reception profiles. As used herein, a reception profile, also referred to in abbreviated form as RX profile, means the intensity of the reception signals as a function of their place of origin in the case of a homogeneous distribution of the tissue properties (such as proton density and relaxation times), as well as in the case of homogeneous distribution of the basic magnetic field and the flip angle.
Conventionally, therefore, the images (after the signals of the individual local coil elements have been combined by a suitable method) are frequently either smoothed by an image-based filter, or are normalized to the relatively homogeneous signal of a volume coil, e.g. the body coil present in whole-body scanners. However, the above-described RX effects result in the signal of the volume coil no longer being homogeneous under certain conditions and therefore having an impact on the local coil images.
The cited use of image filters in some cases likewise proves unsatisfactory, e.g. the entire image impression can be distorted as a result and pathological effects on the image can be influenced under certain conditions.
During the acquisition of measured signals, the reception profile RP(x,y,z) of the reception coil combination used, or of the coil combination to which the measured signal is normalized (the body coil in the above example), is incorporated proportionally into the signal S(x,y,z) measured at the location x,y,z:S(x,y,z)∝RP(x,y,z)  (1)
The effect of the TX profile, which determines the flip angle actually present at the location (x,y,z) during the alignment of the spins relative to the orientation of the magnetic field lines of the basic magnetic field, on the measured signal is considerably more complicated and in general is dependent on the sequence characteristics (e.g. type of sequence, echo time and repetition time) as well as on the properties of the tissue being examined (such as proton density and relaxation times).
As already mentioned, one possibility of compensating for the changed signal distribution is the use of image filters, with the intended object of improving the homogeneity of the MR image acquisitions. This measure is therefore aimed at compensating for the combined impact of the described TX and RX effects. Filters of the type can be designed e.g. to remove low-frequency intensity variations in the spatial domain from the images. These “slowly varying” components are different from the anatomic structures, which generally are composed of higher spatial frequencies. Some image filters exploit the fact that the same signal intensity should be present in specific image regions located at different positions. If a difference in signal intensity is present, then an inhomogeneity in the transmit/reception behavior of the arrangement may be inferred and the inhomogeneity can be corrected accordingly.
Some methods that are used address only the TX effects, e.g. the use of B1 shimming or other pTX methods in order to achieve a homogeneous distribution of the flip angles. These methods assume the presence of a number of transmit channels. In B1 shimming, a suitable polarization is chosen in this case through suitable selection of the amplitude ratios and phase differences between the different transmit channels, the polarization leading to the smallest possible variations in the achieved flip angles. However, in some cases a large number (e.g. 8 or more) of transmit channels are necessary in order to achieve a satisfactory homogeneity. Such an arrangement is quite expensive and usually not available in commercial systems.
Alternatively, it is also possible to use B1-insensitive RF pulses in order to compensate for the TX effects. Disadvantageous aspects with this approach, however, are the longer pulse duration and the increased energy dose, for which reason this approach is not suitable for all MR sequences.